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As a sample is quenched, the efficiency of light production falls, and it therefore

creates an illusion that the radiation has a lower energy. The principle is straightforward:

observe how the energy spectrum shifts as the efficiency falls, store this relationship

in a computer, and then analyse the energy spectrum of the sample to determine the

efficiency. However, there is a catch: different isotopes have different energies so we

need to be sure that we are measuring the effect of quench and not just the funda-

mental differences in isotopes. This is resolved by using an ‘external standard’ source

of radioactivity built into the counter. It’s called an external standard because it is

placed just outside the sample vial by a mechanical device in the instrument. The

quenching in the sample is observed by counting this standard, the external source is

then moved away, and the experimental sample counted. This is done for every

sample in turn and the counter prints out corrected d.p.m. automatically. To set all

this up a standard curve using a set of quenched standards is counted: the absolute

amount of radioactivity in the standard is known and therefore the efficiency of

counting can easily be determined. These are the data that are stored in the computer

as a standard curve.

Some instruments (e.g. the Hidex 300 SL, shown in Figs. 14.4, and 14.8) on the

market calculate counting efficiency in a totally different way. The counters use three

photomultiplier tubes. The mathematics of the process are beyond the scope of this

Example 4EXTERNAL STANDARD EFFICIENCY CALCULATIONS

A scintillation counter analyses the energy spectrum of an external standard, it

records a point on the spectrum (the quench parameter, QP) and assesses the shift in

spectrum as the efficiency falls. The efficiency of detecting^14 C in a scintillation

counter is determined by counting a standard sample containing 105 071 d.p.m. at

different degrees of quench (by adding increasing amounts of a quenching chemical

such as chloroform). The results look like this:

c.p.m QP

87 451 0.90

62 361 0.64

45 220 0.46

21 014 0.21

Then an experimental sample gives 2026 c.p.m. at a QP of 0.52. What is the true

count rate?

Firstly, the counting efficiency of the quenched standards needs to calculated; the

efficiency is the ratio of the c.p.m. to the d.p.m. (100 if expressed as a %). Then the

efficiency is plotted against the QP. The QP for the experimental sample (0.52) is put

into the curve and the efficiency read (in this case 48%). This is then used to

calculate the true d.p.m.: 2026100/48. The answer is 4221 d.p.m.

568 Radioisotope techniques